Is the speed of light constant between galaxies?

I will simply point out that it is far from obvious what it might mean to say that the speed of light is constant.One of the things that Einstein taught us, was there there can be no universal measuring scale. How we measure things, such as speed, will depend on our inertial frame.The question, on whether the speed of light is constant, would seem to require a universal scale.

But I think something is missing there.I was questioning the meaning of "constant speed".As I see it, the constancy of the speed of light is used as a sort of standardizing principle, so as to find ways of translating measuring scales between different inertial frames. It is unlikely that the OP was asking about standardizing principles. So I wanted to know what he thought he was asking about.